Its impossible to tell the difference between (1) a friendly mine (or friendly bubble...etc.); or (2) Enemy mine/bubble/etc.
These things should have different colours: i.e. Alpha team is RED, Other team is Green.
EDIT: Don't forget thaose electric mines which are hardly hidden... just of unknown threat level (friendly/enemy)
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Many times a trip mine has slowed me down cuz I have no idea who's it is