2011 Chevy Equinox - New battery/alternator but no power in the morning

I have a problem with a 2011 Equinox. Went to start it in the morning and nothing - couldn't
unlock doors with fob, no lights on. Can't remove key from ignition after attempting to start.
Jumped it and it started up and ran fine for a couple days. Went to start it again and same
thing - fob won't work, no lights, no dinging sounds, completely dead, can't remove key from
ignition. Called AAA to come and bought a new battery from them for $150. Everything was
fine for about 5 days and then it all happened again. Jumped it, took to mechanic and he put a
new alternator in it. Paid for alternator and went to pick it up after the shop was closed and it
didn't start. Same exact thing - no lights, no sign of life. Called mechanic in the morning and he
"rebooted the computer system." Picked it up at 2pm yesterday and it was running fine. Sat
from 4pm - 8pm and started right up at 8pm. Went to start it this morning and it was dead
again. Jumped it and it the security alarm and radio came on immediately when the cables
were put on the other car. It's back at the mechanic's now, but he is stumped. Any ideas?