Currently, when you spend too much time in an encounter you get to hear a clicking sound
This means that soon, creatures (dogs, mantis or rats) will appear.
I suggest adding:
You hear footsteps
or
You hear somebody approaching
or
You hear a loud fart and one hand clapping
when a player invades your encounter.
You will have as much information as the person joining so you can be prepared for possible combat. The message should not appear if the player travelling on the map is forced into the encounter rather than choosing to invade.
If you miss the notification because you were busy rummaging through a footlocker or looting dead critters, it's your own fault. If you notice the notification and decide to take action, maybe you will drop all the crap that might be overloading you and escape, or reload your weapon and prepare for combat. You won't know who is coming or where from so it won't really give you any real advantage. On the contrary, it makes it more fair against rpk's