Submitted to BugGuide, iNaturalist.ca, and emailed to local pro.
I've been given 2 different "possible" IDs.
Not mentioning the IDs or who ID'ed what to avoid biasing anything (will link below though).
Welcoming any other IDs and opinions (and perfectly aware that this may not go anywhere).
I couldn't get my hands on it well enough to pick it up and out of the grass/moss.
These were the only pictures I managed. I'm inclined to agree with one of the IDs I was given.
Fredericton area - May 30, 2017.
I'm inclined to agree with #2 as I don't see an obvious marker for #1. But I know absolutely nothing of rove beetles.
According to Webster's list Platydracus immaculatus does not occur here; but the colouration fits somewhat. The second choice is close too, but your beetle has a red head, and in the link you provide it is black; but it does occur here in NB.
The second choice also seems to have "eye brows" as a defining characteristic...which mine doesn't seem to have.