Try to notice if the squeaking goes away when you hit the brakes. If it goes away when you hit the brakes then the problem is with the brake pad aligning clips. There are usually two on each brake pad (four per wheel though it may vary per vehicle).
When mechanics replace the brake pads it sometimes happens that they remove these clips and donāt put some of them back. This causes misalignment of brake pads with the disc and causes intermittent touching when youāre driving especially at higher speeds (probably why you notice this on the highway). Lancer has 4 clips per wheel and Mitsubishi packages the front brakes set of 8 clips as one packet.
I myself encountered this squeaking about two years ago and after a lot of diagnosis and pondering (and mechanics saying I need to put in new wheel hubs) the problem turned out to be as simple.
If this is indeed your problem then your brake pads and discs have probably worn out unevenly. The recommended course of action is to also change your brake pads and get the discs resurfaced/grinded/polished. Alternatively, if your pads have only some life left then you may change pads and resurface discs later but in the interim period be prepared for vibrations while braking which increase considerably as brakes are applied at higher speeds.
I hope this solves your issue as I havenāt encountered any other car issues that cause squeaking.