A couple of earlier threads mentioned in passing "temporary wormholes" but I didn't find any discussions on specifics...

Temporary or unstable wormholes could appear and be unmarked, last a certain amount of time and then disappear.

It would not appear in Navigation view but would be visible on the radar once you arrive in the sector.

Players would have to find them and then figure out were it leads to. It may also be asymmetric.

It could present situations such as temporary shortcuts for trade routes, an advantage to schedule a sneak attack, etc.

Sounds good.
They could lead to hidden systems or unexplored space with aliens. :P