Dear Connyh,
From my experience in (wet) fish industry, it is a recommended and routine practice to implement a comprehensive system to block external contamination entering the production area, eg change of footwear, footbaths for general contamination, L.mono etc, use of over-garments. Last time I was in Scandinavia the cooked shrimp factory I was auditing had very impressive one-time usable, paper overcoats for guests, never seen that elsewhere. IMEX, people like BRC are usually quite hot on these types of controls. However, it is also true that I hv seen other (certified) establishments in Europe and Asia with much less rigorous protections, particularly for non-RTE products.
I also suspect these kinds of screenings are probably unreal when compared to factories like packaging manufacturers. Few of the plastic, board manufacturers I hv seen had anything like the above but were nonetheless happily certified to ISO 9001 and were supplying food packaging for name brand customers. I occasionally got the feeling from the responses in some of these establishments that I had been classified as an extraterrestrial nuisance.
With respect to yr original question, i suppose it might depend on the magnitude of an incident in some external office but as long as proper prevention schemes are in use, I would hv thought the risk to the production is 'negligible"
Best Regards / Charles.C