The TB20’s courtesy light is a popular candidate for a flat battery; takes a few days 
Ah, but you have the luxury of a hangar! My last action after sliding the cover over is to peek back inside the darkened interior and look for any tell tale lights glowing…
Aveling wrote:
The one occasion when it did happen is when I left the master on for 3Hrs at an unattended airfield.And yes, I’m very careful about the master switch these days!
Our local trick is to always leave the beacon switch on. Then, if you forget the master on, when you go out of the plane, you (or a kind observator) see the beacon on.
Strolling nonchalantly into the FBO at some far flung New Mexico place, a guy coming the other way stopped me and asked “Is that a rental plane?”. “what’s it to him?” I thought, but answered politely “Yes”. “Oh”, he said, “because you’ve left the master on”.
Aircraft operating in the US are required to have the beacon on and many have it hard wired to the master switch. Not so here, and in the Warrior the beacon light is part of the strobe system which we are trained to turn off in the vicinity of personnel.
(Actually, this chap was asking because he thought the Hobbs would be running and wanted to help save me money! Actually the Hobbs is linked to the oil pressure switch in most planes nowadays, but that’s little consolation when returning to a flat battery.)
On Pipers and Jodels I touch the stall warning sensor as I walk away.
