Because sentry.io is a commercial application monitoring tool which has zero incentive to any kind of application monitoring on non-paying customers. That's just costs without benefits.

You now have to argue that a random third party is using and therefore paying sentry.io to do monitoring of random subdomains for the dubious benefit of knowing that the domain exists even though they are paying for something that is way more expensive.

It's far more likely that the NAS vendor integrated sentry.io into the web interface and sentry.io is simply trying to communicate with monitoring endpoints that are part of said integration.

From the perspective of the NAS vendor, the benefits of analytics are obvious. Since there is no central NAS server where all the logs are gathered, they would have to ask users to send the error logs manually which is unreliable. Instead of waiting for users to report errors, the NAS vendor decided to be proactive and send error logs to a central service.