Do you know for certain its your husband that has done this? Has any other person had access to your car and does your husband normally take your car to fill it up or was this abnormal behaviour - because as you have suggested, this would be a good time for it to be intentionally planted? And was your car used to transport his luggage to enable the tracker to fall out?
Only you know the state of your marriage and whether he is controlling as opposed to protective. He should have told you his intention jf he was being protective. Any chance another family member or "friend" did this.
Personally I would take one of two actions depending on answers to the above. Report it to the police and tell them you fear your car is targeted for theft or worse. If they ask why you haven't mentioned it to your husband, tell them its because you don't want to worry him. No matter the outcome, if it is him, it will be recorded what he has done in case its needed in future
Or maybe go park outside the Police station for a couple of hours then text your husband and tell him you have had an alert about the tracker so have reported it to the police (but haven't really). Tell him it never crossed your mind it was him as a) you trust each other so he would have mentioned it and b) you got the alert when he was out of the country. You hadn't said anything as you didn't want to worry him but now its in the hands of the police, you can tell him with confidence you are safe. Then see what the reaction is. Do this by text and not call so he can't misconstrue whats been said to others.
Stay safe, tell a close friend or womens refuge what is going on as a fallback.
Check your phone and computer for tracking.
Finally, and I am sorry to say this, but given he has left you to go on holiday at such a difficult time, he is clearly very selfish and sometimes when someone is cheating/even casually messing around, they project the potential to do this on their innocent partner and act as if you are the concern and monitor your behaviour.
Please let us know the outcome. Big hugs