Went to a tunnel entrance to check how things were, only to dicsover that a tech was busy clearing my mines and laying his own !@#$\% We faced off with pistols, and damaged each other enough to withdraw, but I'm glad I caught him in the act or I'd have stumbled through his field thinking it was mine

Maybe it's obvious, but never forget to target mines with your mouse to check whose they are, before walking near them hehe.