Do not mention that you saw an open; it can sound intrusive, and the open may have been a machine. One or two follow-ups are enough. If your email is tracked, the recruiter also sees a short notice in it that it contains tracking.
HawkOpen labels likely scanner clicks, for example a click a few seconds after delivery, so you do not mistake them for interest. On the Pro plan you can set a reminder for yourself; on Business, Safe Follow-up drafts a polite follow-up that you send yourself.
Where rules that require consent apply, tracking may start only after that consent has been obtained. The link in the message lets the recipient withdraw consent later.