Cold Water by Dave Hutchinson

To uncover the truth, you must risk everything
When Carey Tews retired from Les Coureurs - the clandestine organisation of high-risk smugglers - she swore she'd never go back. Her cover in Hungary was blown, and even if she could have returned, she wouldn't. That is, until an old friend and lover is found dead in mysterious circumstances.
Back for one last job in a Europe fractured into a hundred tiny principalities, with civil unrest and political instability the norm, she must navigate local authorities, rogue operatives and Russian spies.
What she doesn't know is that the investigation will take her to places she couldn't even imagine.