hlt1slim test fails occasionally

For example this instance failed

Traceback (most recent call last):
  File "/workspace/build/MooreOnline/MooreScripts/scripts/testbench.py", line 146, in <module>
    emulator.check_for_orphans([a["args"][0] for a in task_instance_args])
  File "/workspace/build/MooreOnline/MooreScripts/python/MooreScripts/testbench/emulator.py", line 346, in check_for_orphans
    raise RuntimeError("Found already running processes:\n" +
RuntimeError: Found already running processes:
TESTHLT1SLIM_LBBUILD303_HLT1_0 (8751)
TESTHLT1SLIM_LBBUILD303_Writer_0 (8765)

It's not clear to me why this happens. There is one test job per combination of project and platform and the tests run in a container.

Could it be that the check is sensitive to processes outside the container and is thus giving out false positives?

/cc @raaij @clemenci