I was asked to chime in on this thread and I can see there are many examples of what people think should be done.
What Jim Wyatt indicates is used by some divers and can be recognized as a unusall communications signal in a cave system. One that may suggest a question of other dive teams and or your buddy (The clip suggesting the direction of the searching diver -the clip is non-directional to the line like an arrow but can be discussed amongst the team for its use as a emergency indicator of direction). This method is used most likely in the event of a LOST LINE scenario where you tie onto a secure anchor point and then search for and locate the line, once found - you secure onto it and the clip tells your buddy which way YOU went. In the dark lost line scenario you are off the line so is your buddy so think about it? It is also not a internationally standardized signal amongst ALL the cave diving instructional agencies. There are more then five agencies out there teaching cave diving and they are not all from Florida. IF I personally see this then I woud think of it as a direction a diver went but I was trained in Florida. In the LOST LINE scenario it is not one I personally teach but I would undertsand it if I saw it.
In LOST DIVER you start by first just looking about, reference the line, cover your light if need be, confirming your gas and deco time requirements, determine your search constraints and then PLACE a line arrow Then you tie onto the anchored line arrow point and start your lost diver search governed by your constraints . Your arrow MUST be in the direction of the EXIT. This arrow as Jim has suggested can be left behind to communicate to the buddy that you have searched and left the cave. Your buddy should know your arrow colours and markings (Where are you going to meet after exit? My group meets at my Van) If your buddy finds the arrow while you are searching then you have success, he just tugs the line gently repeatedly or flashes his light to gain your attention, you rewind the line and conduct the dive accordingly to your gas and time constraints.
You search and no diver found you depart with your reel leaving that arrow hoping your diving buddy will see it and know what it means.
So let me comment on what I have read so far and what I think.
First of all if you are cave diving you should be of the attitude that you are preparing the dive solo like and your buddies are there for the fun of the dive, the after dive discussion, and the beer that follows that night. Diving is a solo responsibility conducted as a social activity.
Second of all if you are looking for a lost buddy your buddy should be looking for a lost buddy, We call that having AWARENESS and TEAM ALERTNESS. And it is a fact that you are both lost at that point.
Third, You and your buddy should never be any further then a third of your gas in a cave system and all should have sufficient light sources to exit. I will not leave any equipement less a marker to suggest to my buddy I am looking for him/and or exiting the system, based on remaining gas supply and deco obligation. A Marker is not equipment such as a light. Wet notes are up to you and your team. Each buddy has three lights. If you leave a back up light especially after having a reduced gas supply from a resulting buddy search you have further reduced YOUR SAFETY.
I would not even leave a reel except for:
1. after repairing broken line, and then even questionable after I repaired it, or
2. a blacked out lost line search. It is tme consuming to retrace the line and keep track of the main exit line to make that exit. You're in the dark get out, come back later for the reel. have a back up safety reel (spool) to deal with any possible further emergencies you may encounter exiting.
Your buddy has the correct type and numbers of equipment, you should not be diving with him otherwise. LINE arrow $1.00 USD. A Back Up Light when you need one can be priceless!! A fourth (third back up) light Ok(?) but how much do you want to carry? Are you doing exploration or just site seeing diving. Judge your equipment needs to the dive plan as required, as a minimum three lights: one prime two back ups per diver.
Additionaly to this while the back up light on the line may be to headge the bet for your buddy exiting in a black out scenario to his favour you are making the assumption that your lost buddy is behind you in the system and will come upon the light and be able to turn it on. Not knowing your equipment (possibly adding his stress levels and breathing rate) and the luck of even coming upon the light in pure darkness is a slim chance. If you leave the light on as a beacon what is to say your buddy is exiting already and not going deeper looking for you and now you have just reduced the burn time of that light. LED or not. And what if you left that light there in a silt out turned on or not would it really help that other diver?
I believe the line arrow left behind, Team awareness and alertness and a smart head on your shoulders are all that is required to permit YOU to safely exit the cave in tact. Your buddy close behind or ahead of you.
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sorry couldn't resist 
