Those Holz-Her belt sanders & their brethren from all former licencees have fairly generous grease packing of their bevel gears, just as do most well-manufactured angle grinders for that matter. Generally the geartrain in all these machines is "sealed for life", but on occasion some ullage occurs. Sometimes from heat (all friction of any sort will generate heat, even the best lubricated) & sometimes from differential air pressures.
Don't sweat it. All is good, unless of course the seepage becomes a flood! It may just be down to careless grease filling or assembly from the production line. It's not like those crappy Kapex saws that on occasion literally drip gear lube! The Holz-Her belt sander design is a long-standing (40 year old?) quality tool that was actually designed by one of Germany's premier tool manufacturers, the design being used and licenced at one time by many of the very best German tool makers. I suspect this design was one of the primary reasons why Festo actually acquired the company!