1. either their actions are ignored by the victim states, which makes this very cheap and effective sabotage to weaken EU members.
2. or alternatively, if there's military retaliation, putin can finally do the mobilization he promised not to do for his war on ukraine, because he can now assert that the country is under attack by foreign nations.
the EU / the member states are now in a pickle: they know it's russia but can't say it out loud because then they'd have to take action. they also can't do nothing because then russian sabotage could wreak havoc with the population and economy. so what can they do?
1. military support of ukraine 2. further sanctions of the russian economy, which includes getting rid of dependencies on russian goods 3. re-arming and modernizing their own military (i.e. introducing modern drone warfare capability, launching their own satelite intelligence, ...) 4. start opposing/controlling russian influence by local malicious actors, i.e. russia friendly right wing political parties (which is somewhat complicated but not impossible in a democratic setting)