What happened?

Health Physics responded and found that the meter readings were slightly higher near the back wall of the bench. The wall itself had even higher readings. The source of contamination was found on the bottom of a cabinet hanging over the workbench.  After discussion, it was determined that a radioactive sample was placed in a cuvette in the heater block on the bench. The sample boiled over and popped the cuvette lid, shooting radioactive material up onto the bottom of the cabinet.

How can incidents like this be prevented?

Radiation is emitted in all directions from a source, but the Geiger-Muller counter is sensitive in the plane of the probe.  Always search for the highest reading on the detector to locate the source of radiation. If elevated readings do not change over an area, rotate the probe 90 degrees to determine if the source is on another plane.