I frequently fly out of both Newark and Phily airports and have not had any problems at all (but I am sure I just jinxed myself for next time). I use the long term lots even when I'm only gone overnight because I figure there's less people moving about than in short term or hourly.
I do what I do anywhere... park as far away from other cars as possible but without being the only sitting duck in the area... try for an end slot... don't park next to a hooptie or a mini van or SUV... and hope for the best. Good luck
(or take a car service if youre that worried, the $ may be worth the piece of mind)
'14 528-xdrive - daily driver/winter beater, '06 Boxster - for fun, '07 M6- for even more fun.