For a place that welcomes the public or handles food, finding a rat mid-service costs far more than a regular visit: stock thrown away, customers lost, even an administrative closure. Regulation (EC) No 852/2004 on the hygiene of foodstuffs also requires food businesses to have pest control procedures in place.
Year-round monitoring sets up a permanent system: numbered monitoring stations on a site plan, visits at fixed intervals, entry points gradually sealed and a log kept up to date. If you notice activity between two visits, an extra visit is included in the contract: that is what active vigilance means.