Are we talking maintenance workers bringing in metal shavings of stainless steel that they had cut in a shop? If so I'd say there's a few things to look at.
1) You should have a procedure that verifies any maintenance work is inspected after being completed. The areas worked on should be cleaned and sanitized if needed and released by management/quality personnel. That verification should include visual inspection of foreign material.
2) Is this amount of metal that is possibly on someone's clothes a concern of significance? A risk analysis should be done possibly to see if this is of legitimate concern. To me it seems like small metal shavings present a very low risk (most metal shaving wouldn't even register going through metal detection and as a standard most pieces of metal/foreign material aren't considered high risk unless the size is that which could cause a safety concern)
3) If you do really find that there is a high risk of contamination you may want to add a procedure for maintenance to have an extra over coat to use when working in production areas with exposed product. Maintenance guys are usually full of grease and other contaminants and we had a specific program that they had to wear a dark maintenance jacket over their regular shop clothes if there was concern of product contamination. If you have foot wash procedures and you worry about metal shavings coming off the floor that might be a whole other issue because there shouldn't be a concern over product contamination from materials coming from the floor.
Just a few things to think about there but first off I would guess a risk analysis should be done to see if this is really a concern. To me it doesn't seem like anything that a quick visual inspection after maintenance work wouldn't clear up and put it as no risk to product contamination.