@Martin Haustein - Do you have an Enterprise subscription? if so, I would check with the admins of your plan.

today some of our visitors got a message

If the “message” was in the form of a prompt on a Miro board, and not an email, then perhaps someone created a board using Miro Lite (www.miro.lite). For example:

We are only on starter plan at the moment and I am an admin of that.
The message was indeed a prompt on the Miro board, but was not created in Miro Lite, the board has been around for about a year already.
 

Two more questions:

1. when you open the board and click on the board title to open the board details panel/window, does it show that it is in your Starter Plan team? This may seem like a silly question, but the board could actually be in another team and has been viewed using “visitor”/Anyone with the link access this whole time.
2. Another scenario is that the messaged is prompted to users who are on a trial license for the team (I believe the current trial length is 7 days).

1. Didn’t even realize that could be the case, but the board is indeed in our starter plan team.
2. The board has been used mainly by one visitor for about as long as it existed. (a year give or take), they are also the one that got this message.

At this point I am really just wondering about something. If a board is not opened for an extended period of time, does it mean that external link to it could expire? We got some legacy boards where that could be a problem.

@Martin Haustein 

At this point I am really just wondering about something. If a board is not opened for an extended period of time, does it mean that external link to it could expire?

There is a public/visitor link expiration feature, but only for Enterprise Plan subscribers.

Is it possible that your organization has upgraded?

A few indications of this would be:

As per this help center article:

Public link expiration
If you see a clock icon in the share window, public link expiration has been enabled by your Company Admin. Any public links shared with Visitors will automatically expire when a board hasn’t been opened for a set time. If you're no longer able to access a board, it's possible your link has expired. Try reaching out to the board owner and ask them to share the link again.

https://help.miro.com/hc/en-us/articles/360012524559-Collaboration-with-Visitors#01H8KC61MTPJ5E3HV929Z46B5A

And more from the Enterprise Plan help article:

https://help.miro.com/hc/en-us/articles/360017730133-Sharing-policy-on-Enterprise-plan#h_01FB3ZSEF85VKJ03YAG488T998

And you would also be seeing the “organization/company” icon on the team space in your dashboard:

And if the team in which the 24-hour-expiry-message board resides is definitely not a part of an Enterprise Plan subscription, then I would suggest that you report this to the Miro support team for further investigation. You can open a Miro support ticket by following these instructions:

https://help.miro.com/hc/en-us/articles/360020185799-How-to-contact-Miro-Support#h_01F010RXA0E1S9C0WFQYX75R6C

Well I know you can control this feature in the enterprise plan, I was thinking more along the way if there is some default function in other plans that works like this.
And yes I am pretty sure we are a starter plan.

Other than that, I am out of ideas. It seems like the board access didn’t in fact expire, so it seems to me the warning message might have been a bug.

@Martin Haustein - Yes, it could have been a bug/glitch. Since you have a paid subscription, if it happens again, or you wan to report it to the support team anyway as feedback/an FYI, you can open a Miro support ticket by following these instructions:

https://help.miro.com/hc/en-us/articles/360020185799-How-to-contact-Miro-Support#h_01F010RXA0E1S9C0WFQYX75R6C