Could always try to implement a map sort of like in the game state of decay were the world carries on while looking at the map so it's more dangerous to use one, also what you could do with the pen idea is to have a few different marks/symbols ie. Crosses, ticks, circles, and many more but each symbol does not have a specific meaning, as in you decide which mark is for what, for example you could pick to use a cross to mark a place you have fully searched, but your friend could have used a circle ....just a small idea to make it feel like it's more of a personal map rather than a generic forced map options, also the map could not be found all in one piece (apart from the odd very VERY rare occasions) so by this I mean you find teamed map pieces which you use tape to fix the map pieces into a whole
