Originally Posted by Armiger
Nothing like you describe is shipped in the case by Glock. Since you referenced "stored" for a few years, did you buy it used from an individual seller in a case and then stick it away? If so, it likely from your description it is a snap cap and was chambered.
Armiger is right, no matter what it is, it didn't come from Glock. The fact you had difficulty working the slide is also disconcerting, a Glock should be able to sit for many years and function flawlessly.
If you're not comfortable doing a detailed inspection yourself, take it to someone. It's not a big deal to inspect.
BTW - Outside of the frame, slide and barrel you can replace almost everything else in a G17 for about $60 so if any repairs are needed, I would expect them to be very reasonable.