It actually is a surprisingly big problem that gets little notice. Multiple studies of autopsies have shown that 3-5% of autopsies show the cause of death as pulmonary embolism. And it is about the same rate whether the patient died in a hospital or died elsewhere - so it is by no means exclusively hospital related.
Obviously the set of patients autopsied is the set where they didn't die of obvious causes (e.g. cancer or treatment for stroke). But still that is a much bigger percentage than I expected at around 2% (WAG) of all deaths being pulmonary embolism. It isn't heart attack or stroke - but it would be bigger than, for instance, prostate cancer and about the same size as breast cancer.