I don’t have a PIR to test, but looking at the code, I’d suggest trying something like this:

You need a DOM element to show the message in. You can borrow it from the default module helloworld and modify it:

getDom: function() { var wrapper = document.createElement("div"); wrapper.setAttribute("id", "welcome-message"); wrapper.innerHTML = ""; return wrapper; }

And then, based on self.sendSocketNotification("USER_PRESENCE", true); in the node_helper.js you add
this to the MMM-PIR-Sensor.js :

socketNotificationReceived: function(notification, payload) { if (notification === "USER_PRESENCE"){ this.sendNotification(notification, payload) // new: if (payload === true) { document.getElementById("welcome-message").innerHTML = "Welcome back!"; setTimeout(() => { this.removeMessage(); }, (1*60*1000); // = 60 seconds } }, removeMessage: function(payload) { document.getElementById("welcome-message").innerHTML = ""; }

If you use it like this, the message is only shown for 1 * 60 * 1000 miliseconds = 60 seconds (set as desired) and then removed.

This is all untested but hopefully gives you some clues where to look and work on the code.