A B737-400 FO RPTS NOTED WINDSHIELDS FROSTING OVER. FOUND WINDOW HEAT SWITCHES IN 'OFF' POS. BELIEVES WINDOW HEAT TURNED OFF DURING MAINT PERFORMED BEFORE SECOND LEG.

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IN CRUISE FROM ZZZ1 TO ZZZ WE NOTICED THAT THE WINDSHIELDS WERE FROSTING OVER SO WE CHKED THE WINDOW HEAT SWITCHES AND FOUND THEM ALL OFF. THIS WAS OUR THIRD LEG IN THIS PLANE AND WE KNOW THEY WERE ON FOR THE FIRST LEG AND THAT NEITHER OF US HAD TURNED THEM OFF AT ANY TIME. BTWN THE FIRST AND SECOND LEGS WHILE ON THE GND IN ZZZ; MECHS CAME INTO THE COCKPIT TO WORK ON VARIOUS SQUAWKS. THESE INCLUDED RE-LAMPING ONE OF THE PITOT HEAT OFF LAMPS NEAR THE WINDOW HEAT SWITCHES; JUMPSEAT OXYGEN MASK/SMOKE GOGGLES AND ALSO HAVING THE WINDOWS CLEANED. THEY MUST HAVE TURNED THE WINDOW HEAT SWITCHES OFF TO CLEAN THE WINDOWS. THEY NEVER TURNED THEM BACK ON; AND WE NEVER THOUGHT ABOUT THEM TOUCHING THE SWITCHES FOR ANY REASON. OUR THROUGH-FLT PROCS DON'T REQUIRE RECHKING WINDOW HEAT AND WE DIDN'T SEE IT. IN MY MIND; THE MECHS DID NOTHING THAT WOULD AFFECT OUR SYS SO I HAD NO REASON TO REDO THE COMPLETE COCKPIT FLOW AND CHKLIST. I HOPE THE MECHS ARE REMINDED TO NOT CHANGE COCKPIT CONFIGN ON THROUGH-FLTS. THIS IS POTENTIALLY VERY DANGEROUS.