Lower case not sitting flush on side

After getting my MacBook Air back from getting serviced (trackpad broke) the lower case has not been sitting flat on the left hand side of the unit (MagSafe, Headphone and Mic side). It's not too visible but when you run your fingers over it the lower case sticks out by about 1mm. It's fine on the other side.

I went ahead and ordered my own screwdriver in order to try and tighten the screws but no luck, the screws were already in fine. I then decided to take the lower case off and put it back on again with no luck still.

It flexes a tiny bit around the edge when you squeeze on it, about the same as the other good side.

I'm not sure if the lower case is warped or something's been installed wrong (battery not in properly?). I don't have the tools to remove the battery right now, only the lower case (not sure why they needed to make the inside parts torx).

The repair was around 6 months ago and I've put up with it since then. My warranty ends in a few weeks so it's now or never. There are no Apple stores around here so my only option would be to bring it in to the place that replaced my trackpad (under warranty with Apple's approval).

I'm willing to DIY (why else would I post here) but first I need to know where the issue lies. Warped case or loose battery are at the top of my list.

Edit: I've contacted the place that replaced the trackpad initially. See how that goes.