I have the 945 bundle (that includes HRM-Swim and HRM-Tri). I lap-swim with the HRM-Swim.
This issue just happened to me this morning after swimming for a month with my watch, during which it wasn't an issue. I will see if the issue returns tomorrow morning.
Basically after my laps were over, the watch was doing it's usual "searching for external hr data" thing, and it couldn't find it on about 5 attempts. Then I got fed up, and selected "skip" (in other words, don't use external HR data). After the "skip" it the HRM-Swim was immediately connected and HR data was downloaded.
Speculation 1: It might be related to the fact that now the watch can use HR data during swim without a strap as well.
Speculation 2: The watch might have been trying to find the HRM-Tri strap, and the error relates to it.
Has anyone had similar experience?