If you are not leaking oil, it can be a seal in the trim pump valves, or it could be the O-ring internal to the trim cylinder (either one, does not have to be both).
As deafwish implied, if it's not leaking oil, you can wait for it to get worse before addressing the problem. Lowering over the course of "a couple weeks" is not too bad, but if you are noticing the trim creeping down while you are cruising, then it's pretty bad.
It's not too hard (or expensive) to replace all the O-rings in the trim cylinders DIY, but you need a spanner wrench to unscrew the cylinder head.
_________________

"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings