... just bring the security guard a doughnut every now and then!

I mean, really, a guardhouse a few feet away? I think you're covered.

I leave my bike at a bike rack in our office garage, with a uniformed guard and a very busy door about 30 feet away.

I secure my bike with a bogus lock -- one of those cable jobs. Other riders (we have about three, sometimes four, out of 2,000-plus employees; kinda sad) don't bother with a lock at all.

I leave my lights on the rig, and often leave my helmet and gloves there, too. Nothing missing.

I would not, of course, leave it there overnight. Guard goes home at 7 p.m., and the garage is not locked (in fact, doesn't even have doors).

Sounds like you've got a good situation. Keep the $15.