Isn't there something that informs the pilot that the plane is missing a tire/wheel or that it weighs less than it should?

Nothing I know of for a missing . . . or the plane is lighter than it should be.

Of course they have sensors. I have to research this a little more carefully. The article itself seems iffy. The headline reads Miracle as plane lands safely at Heathrow despite losing a WHEEL ****before take-off**** without anyone noticing” But in the body it says it was lost mid flight.

The weight changing would be useless, unless it was weighed after it was fully loaded and before takeoff.

I remember when the Space Shuttle broke apart on reentry, one of the first sensors to alert NASA of a problem was a tire inflation sensor.
I doubt passenger flights have that.