Facetime Unknown Caller late at night

The fact there were no details available to you (beyond the fact you could see their number) simply means they're not in your contacts - it doesn't necessarily mean the caller was unknown to you.

As for how they "received your number", well, anyone can call any random number and potentially it'll belong to someone. That is nothing new and has been going on for decades. Likewise with emails - I get emails from unknown people all the time. It doesn't mean they 'know' me and my email address.

It's no different for FaceTime. A caller could try their luck and FaceTime a random number or a random email address and if they belong to an iPhone user or to an Apple ID, well, the FaceTime call comes through.

So, if you don't want an incoming FaceTime video caller to potentially see you (or your home, etc) then the best option is simply not to answer it. If it turns out to be someone you know, they'll try you again later.

However, you can silence unknown callers to your iPhone's cell number. for more info on this and other options for avoiding unwanted calls to your cell number, refer to Avoid unwanted calls on iPhone on Apple's website.

You can also block unwanted callers in FaceTime on iPhone (but this is not the same as 'unknown' callers).