If you implemented the laws of physics on a computer, using lazy evaluation, then whatever is "over the horizon" from the observer process(es) would not be computed.

However, this would not in the least be observable from inside the system. If the observer moved to serve you, your past would be "retroactively" computed.

I'm not claiming this is very likely to be the case, since at the very least it requires an additional agent - the observer process - to cause anything to happen at all, but lazy evaluation isn't some weird ad-hoc concept; it's a basic concept in computer science that also happens to make programs shorter, a lot of the time.

Hopefully not sufficiently shorter that a universe using lazy evaluation with one random point in space somewhere as the observer is less complex than one using strict evaluation. That.. would be impossible for us to detect, of course, but I believe it'd still have consequences.