The signature will detect the instance but will not report the discovery path. If discovery path is needed, then the following should help:
<MultipleInstance>
<Iterator export="true" name="INSTALL_PATH">
<Vector filter="*test*">
<FindFilePathEx name="blah.sh"/>
</Vector>
</Iterator>
<Instance>
<Variable name="IS_INSTALLED">
<FileExists absolutePath="$(INSTALL_PATH)blah.sh"/>
</Variable>
<Condition withVariable="IS_INSTALLED">
<Action do="SKIP" onValue="false"/>
</Condition>
</Instance>
</MultipleInstance>