I fully agree with manfromplaid.
In addition, I have the Signal, and it is extremely accurate measuring the ambient temperature around the probe. A couple of things to consider in placing your probe. Make sure you use the air (ambient) temperature probe, when measuring the cabinet temperature. That is the short blunt probe. Thermoworks states that the air probe is more accurate than the meat probes; for this measurement.
The placement of your probe is also important. Lowering the probe so that it hangs in the middle of the cabinet, may not give you the most accurate reading. I always attach my probe to the lowest tray - farthest away from the element, and keeping the probe at least a couple of inches from the meat. When food is cooking, it releases moisture through evaporation. Evaporation can cool down the area around the meat by as much as 20°F. Or in your case, above the meat, as the moisture moves up and out of the vent.
How did your ribs turn out?