I find this all the time on resi jobs. The bottom of the loadcenters get all rusted out. Many times the CB are also corroded, some are stuck in the on postion. Service cable gets a little hole in it and lets the rain in, cable assembly acts like a hose, drips down into load center, sometimes days after the rain. The other one I see alot is dried duct seal, water drips over the lugs in the meter down into the lower cable into the load center. Weep hole will not prevent this. It will make it more noticable, when water drips on the floor.
Choose your customers, don't let them choose you.