The Falling Leaf maneuver is a way to lose altitude by descending like a tree leaf. Is this maneuver used in the recovery of UAVs?
The maneuver as described is not a particularly efficient way of losing altitude: You're alternately stalling & recovering from the stall.
More efficient descent procedures exist.
The technique of holding the aircraft in a wings-level stall (or putting it into a spin) was used in the early days of aviation to get back below a cloud deck if you were trapped "on top": when you broke out below the clouds you would recover from the maneuver & proceed to your destination / landing (if you were lucky enough to have sufficient altitude to recover when you came out of the bottom of the clouds).
Needless to say neither of these maneuvers makes much sense for an unmanned aircraft: If your goal is a rapid descent you will execute a maneuver that gives you the best possible descent rate (such as the emergency descent procedure in the POH, or the one in the Airplane Flying Handbook); If your goal is to get back below the clouds you've already busted the regs (you need to maintain visual line-of-sight with your UAS), but if and when the regulations change you should also be able to recover using more modern techniques (like descending on an instrument approach or using a synthetic vision system) to ensure you don't hit anything, like another aircraft.
I use falling leaf with my suas when I want to loose altitude quickly, and don't want to risk entering vortex ring state. For me, I can drop from 350ft to 30 ft reasonably quickly, and feel that I have full control using line of site. After reaching about 30ft, I can easily pull out of falling leaf and do a nice slow landing.
While not a "Falling Leaf" Maneuver, a "Deep Stall" to landing is actually a very common way to recover a UAS. This is the default behavior for RQ-11 and RQ-20. The systems are designed for this and absorb the energy of the fall in padding and by breaking apart. Each system has a spares kits for wing panels and nylon fastners. An advantage of this method is that it improves accuracy by simplifying energy management; just fly over the intended recovery zone and stall it in. The vehicle will be close to where you wanted to land.