Law 18. Home team is responsible for the safety and security of the site. If you can't identify the affiliation of the miscreant, then it falls on the home team to fix the problem just as it would if the goal wasn't secure.
Absolutely! I carry my cell phone on the field in some of these situations where I feel it is more than unlikely that I might need HELP in solving a problem. I don't like being in this situation but I'd rather call 911 than watch something go down that can't be stopped. (This hasn't happened to me recently but I've seen it and don't want to see it again) "Law 18. " Always!