We study how sequestration mechanisms, such as protein binding to decoy sites and the reversible formation of phase-separated condensates, alter the statistics of protein levels. To implement such a type of regulation, we consider simple birth-death processes in which protein molecules can reversibly switch between active (free-molecule) and inactive (sequestered) configurations with arbitrary concentration-dependent rates. When these transition rates depend only on the levels of the inactive protein, we show analytically that the steady-state distribution of the active protein level has Poisson statistics regardless of the details of the sequestration process. When transition rates depend on the active protein, the molecule copy number distribution is non-Poisson. We illustrate this deviation with the example of an enzyme-driven post-translational modification of an active protein into an inactive state, and we confirm the results using both analytical approximations and exact stochastic simulations. In summary, our results characterize the role of sequestration mechanisms in modulating the stochasticity of biochemical processes and identify noise-invariant processes that we tie to specific biological mechanisms.