if the seals were leaking, you would be able to tell. there's enough pressure in the system that if there was a leak in any rubber parts (lines or seals), it would open up pretty quick and you would feel a noticeable change in pedal pressure.
as for the reservoir, the level naturally drops as the brakes are used up and the calipers adjust. unless you lost a lot of fluid, it's doubtful they would have added any. you just don't want to let it get so low that air is able to get into the lines.
I wouldn't stress over it too much unless your pedal goes to the floor.