Do you think the management of Rabobank are just going to keep quiet and let everyone think they're Rabobank employees? The first opportunity they get they'd make a public statement that the offices were occupied by Marsh Insurance. Keeping quite just makes the management of Marsh look cowardly and deceitful, not a good look for a company that relies on trust.

Secondly, Rabobank pays the owner of the building a substantial amount of money for the naming rights to that building. Somewhere in that contract will be a clause to the effect of "The building owner will not allow activity in the building that could negatively affect Rabobank's public image". The primary intent would be that the building owner can't lease out the top floor to a brothel, or turn the basement into a strip club. Rabobank may even have right of refusal regarding building tenants. Depending on how it's worded, Rabobank may have cause to sue the building owner, and the building owner may have grounds to sue Marsh Ltd, or cancel their lease.

Another factor is that Rabobank lends people money. Most large purchases also require insurance. (last time I checked banks won't give you a mortgage unless the house is insured). Marsh Ltd possibly gets customer referrals from Rabobank. Screwing up that professional relationship could cost them millions.