What actions might invalidate cover?

For all indemnity policies, it is a condition that their existence must not be revealed to third parties. The disclosure of the policy (a) suggests that the insured’s position is weak; (b) may indicate to a third party that they have rights of which they were not fully aware and (c) the claimant may think that the insurance company has lots of money and is afraid of publicity, so will pay rather than argue, thus giving rise to increased costs in dealing with the claim.

If a third party is made aware of their potential rights, insurance cover is unlikely to be available against the enforcement of those rights by that third party. If you are acting for the buyer you may prejudice the seller's position.