Why is "estuviera" used here?

Why is "estuviera" used here?
 

I have the following passage:

"Ferrán y Pascal corrieron hasta llegar a la cubierta, se quitaron las chaquetas, con lo que pretendían despistar a quienes los siguiese, y se dirigieron a la popa, donde tenía que estar Marta esperándolos para repartirse el sabroso botín. Esta sintió un inmenso alivio cuando los vio llegar caminando tranquilamente, sin que nadie los estuviera siguiendo y con el resto de los pasajeros que paseaban por la cubierta sin sospechar nada de lo que acababa de ocurrir."

I don't understand why "estuviera" is used. To be clear, I understand why the subjunctive is used, but not why "estar" is used in general. I feel like without "estuviera" the phrase reads "without anyone following them", but with "estuviera", it reads "without anyone were/was following them". The only thing I can think is that since there's no progressive subjunctive, "estuviera" had to be added to supply the subjunctive mood. Is that right or is there something I'm missing?

I would expect to see «sin que nadie los siguiera». The use of the imperfect subjunctive is required, as you know, and it means what you wrote.
To me, there's no justification to use only the participio «siguiendo».

Wait for a native speaker to chime in, but I think «estuviera siguiendo» is overkill. The sense of "progressive aspect" is conveyed by the imperfect subjunctive.

I agree with Rusty that it is not really necessary to use "estar siguiendo", but it's not strictly incorrect. I'd say it's written in some form of colloquial speech, for which is a valid construction. If it were more formal writing, then it would not be completely appropriate. :)