Three weeks ago, en route to Sevilla on the morning EVA bus from Lagos, about 5 minutes from VRSA, and with three pax of Asian looks on board, I heard the driver phone the police at the border to say the bus would be crossing the border in "15 minutos"!
When we arrived at the Guadiana bridge, sure enough, there were two Spanish Guarda Civils from the joint border control point waiting to walk through the bus and check ID's. The three paxs turned out to be from South Korea, so no problem.
On the return trip, a week later, we passed over the bridge with no id check stop.
I have noticed the drivers doing something similar before but have never been close enough behind them to hear the conversation exactly. It appears the the drivers of both the DAMAS and EVA buses on the Sevilla run have an "unofficial" agreement to notify the border police if they are carrying passengers whose id they think should be checked.
Therefore, always carry a passport or Euro ID if crossing the Portuguese border between Portugal and Spain at VRSA/Ayamonte.
