They're not thrown anywhere, either: no bottle ever hangs in mid-air, no rifle sticks out of a wall. Valve's Michael Booth showed why in his 2009 talks. "Map designer creates many possible item groups throughout the map, the AI Direction chooses which groups actually exist." The same for the big guns: "the AI Director chooses which will actually exist." Why let a designer place them? "Allows visual storytelling/intention" and "Solves item placement issues (leaning against wall, mounted in gun rack, etc)." And: "Prediction of possible locations beneficial in this case." You can learn where they might be, never which.

One exception, from Booth's answer at GDC: when the team is down and nobody has any health, the Director "can change pain pills into health kits".

And why a bathroom? In the game's own developer commentary, Valve's Phil Co says the areas off the main path were built for the Infected to spawn in. Players "naturally wanted to explore them", so "fairly late in development, we decided to reward the players for going into these areas".

The apartment, the figures, the nine spots, how many the Director keeps each run and the stacked runs are our own simplified version, built in code from Valve's words. Not affiliated with or endorsed by Valve.

Which room did you always check first?

Sources: Michael Booth, "The AI Systems of Left 4 Dead", Valve, AIIDE 2009 (slides 64, 65, 75); Michael Booth, "Replayable Cooperative Game Design: Left 4 Dead", Valve, GDC 2009 (slide 65 and the talk's Q&A); Left 4 Dead developer commentary (Phil Co), Valve, 2008.
Sounds and music: our own, made in code.