There is, the subject is die Polizei.
In German, the subject in declarative sentences is often found in the position to the left of the finite verb (pre-field): Die Polizei kam. Sie spricht Deutsch. These are the sentence structures that one would typically encounter in a beginners' class.
However, that is not a necessity (there are studies suggesting this is only true in about 50% of the cases). Many connectors can take this pre-field position as well, the reason typically being that this allows them to more clearly express the relation of the internal ("the statement that follows") to the external connect ("the previous statement"). Take your example sentence: Clearly dann directly refers to a previous statement. There must be some context, presumably some sequence of events (First x happened. Dann kam die Polizei.). On a purely intuitive level, you would perhaps even expect in such a sequence of events that the dann wants to be as close as possible to the preceding event, just to better serve its "structuring" function. And, indeed, dann in this sequencing function is mostly found in the pre-field. (Die Polizei kam dann would be fine in terms of grammar, but for the foregoing reasons, speakers would clearly prefer the other version.)
With dann in the pre-field, you immediately know that die Polizei must move somewhere to the right of the finite verb. There's just no other place to go, you can't squeeze it in between the dann and the finite verb. (This sounds technical but in practice it will become intuitive at some point.)