[jacquesm]

The only borders of note in Europe are the Polish-Russian border, the Hungarian-Romanian border, the Borders with Bulgaria and Turkey. And of course there is the UK. Most of not all of the rest are just lines on maps these day and pose no significant barrier to entry from parties going one way or the other.

There are 'roving' patrols but these are far too few to make any big difference.

I travel a lot within Europe and the borders above are the only ones where I've seen any border activity at all, let alone was stopped for inspection of my vehicle or passport control in the last decade or more.

[gambiting]

The Polish-Russian border is so small it's almost insignificant. There is also very little to no traffic going through it, because there is nothing of interest in Kaliningrad. You must be thinking of the Polish-Belarus border, since most of the European traffic to/from Russia is going that way. Also the Polish-Ukraine border can have incredibly heavy traffic, with waiting queues of couple days at some times.