How are planes repossessed?
Depends on the type. In smaller aircraft, it's not much different than with cars/boats/etc. Companies like Sage Popovich specialize in such.
In larger/commercial aircraft, it's usually a matter of injunctive action. Lessors and lenders write the terms into the contract, and they're generally straightforward:
"If you owe us $XX beyond YY amount of time, then we the lender will seek immediate injunctive relief, and the last place the aircraft lands is where it will remain until we decide otherwise."
Do they just sneak in the airport at night and fly it away?
No. Again, it differs by the type of aircraft:
Smaller ones, may have locks/devices put on them, until an independently designated MRO provider can certify that it's in sufficient condition to be flown elsewhere.
In commercial aircraft, the maintenance log is seized and reviewed, prior to a decision on operation; though most lenders will demand an MRO review as well, just to CYA.