It wouldn't be that big of a deal to reinstall the small cup shaped spacer if that's what you find wrong. In the case I stated, I just reinstalled it and never had the problem again-it was a regular customer.
Maybe the valve stuck in place from sitting just long enough for this to happen-no real firm evidence of why it came out.
Yes, the hydraulic compensator you could think of as being a "lifter", although this term is usually thought of as being a pushrod engine part. Usually the lifter slides along the camshaft. On this engine the rocker arm has a polished curved section that rides on the camshaft, and the compensator is on the other end of the rocker over the valve stem. Keeping the valve lash in spec is the job of either of these parts.
A recommended procedure for checking for a collapsed compensator is to push down on the rocker arm on the side opposite the camshaft (the end of the rocker that the compensator is) and push down with something like a hammer handle and see if the rocker seems to move down alot.
But check just for alot of looseness first, to see if maybe it's just the small button flipped out, or maybe for a damaged camshaft.