放哨 is a separable verb (离合词) meaning 'to stand sentry' or 'to be on guard duty'. The noun 哨 can split off the verb 放 when you need to insert duration, measure words, or other elements — for example, 放了一夜哨 ('stood guard all night') or 放过两次哨 ('been on sentry duty twice'). You do not put the location or time directly after 放哨; instead, place them inside the split structure or before the verb with 在.