There are several reasons these could be on. i assume your mechanic also checked the brake fluid level, but just in case you should look at that, and also make sure the brake lights are working.
This is the simple stuff that will turn them all on.
If that all checks out, then you really need to have it scanned. The reason is there are 4 wheel speed sensors, a yaw rate sensor, a G force sensor that could be the problem.
Let me be clear, the car will still stop and is safe to drive. This is an electrical problem and not a mechanical one. All it takes is one of these sensors to fail to kick the lights on. With so many it's best to have the abs control module scanned to know where to start looking.
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Thank you so much!