Maybe the shop is using a scanner that doesn't have access to vehicle/manufacturer specific error codes? The light being on should show some code if the scanner is able to read it.
If it has to do with the DRL system, a generic scanner may not be able to read it.
If you have an advance auto or autozone nearby, they might be able to read it for you. (Usually, their scanners can read more specific codes).
Might be able to check the fuse for DRL (should be labeled in your fuse box) and see if it is still good. If not, there might be a wiring short. If its bad, could try replacing the fuse and see if it holds. Sounds like the system can't activate the DRL, which throws an error code and turns the CEL on.
Not being familiar with the vehicle, I can't be of much more help.
2004 Buick LeSabre Custom FE5 GXP suspension conversion. 170Kmi
2001 Silverado 4WD 5.3L, 33" BFG ATs, Z71/Z82 pkg, 147kmi.