Hi Henk,
It could be that air has escaped after heating of the bottle and that cooling down did not let any air back in and therefore the bottle caved in.
It seems fairly strange to me that hardener evaporated, because there's not much to evaporate in hardener or in resin. They are not really a volatile liquid.
I think you should still be able to use them. Try a test and if they harden up properly they should be ok.
JohnK

Hi Henk,

: Hi John

: No it is a full 1/2 gallon plastic container, just no seal other than the
: cap. The container was visibly caved in a bit, enough for me to be sure I
: would have noticed it if it had been like this in the store. It also
: happened with System III containers that I thought I had sealed well. Not
: with a Fiberglass Hawaii home brand that came in an metal container. Just
: wondering if something can get through the plastic and if so, what.

: Henk