How do memory cells(B-cells) encounter pathogens?
Great Question.

Location:
Memory B cells are highly abundant in the human spleen, and they make up 45% of the total B cell population in this organ. There is evidence to indicate that human memory B cells might mostly reside in the spleen, and some memory B cells recirculate in the blood [1]. 

Activation:

Memory B cells circulate throughout the body in a quiescent state until specific antigen is re-encountered and triggers a potent secondary immune response [2].T cell help is a strict requirement for the reactivation of memory B cells that are specific for monomeric protein antigens [3]. 

Sources:


Hauser, Anja E., and Uta E. Höpken. "B cell localization and migration in health and disease." Molecular Biology of B Cells (Second Edition). 2015. 187-214.
Firestein, Gary S., et al. Kelley's Textbook of Rheumatology E-Book. Elsevier Health Sciences, 2012.
Janeway, Charles A., et al. "Immunobiology: the immune system in health and disease." (2005).