Dangers of expired bleach?
There is a old paper which discuss the mechanism of decomposition of bleach. It says the major product is chlorate($\ce{ClO3-}$)


  The decomposition of sodiun hypochlorite has been re-examined. The
  results show that Foerster and Dolch's mechanism of the decomposition
  to chlorate and chloride is correct; they postulated a slow
  bimolecular reaction to chlorite and chloride, followed by a faster
  reaction of chlorite with more hypochlorite. [...] The rates are such
  that at 40 C. A solution of sodium hypochlorite will contain about lyo
  as much chlorite as hypochlorite.[...]. No signs of specific catalytic
  effects of sodium chloride, hydroxide, or carbonate could be observed,
  and it seems probable that earlier reports of this were due to
  variations in ionic strength. The decomposition to chloride and oxygen
  has been measured and is a unimolecular reaction, which is possibly,
  but not certainly, uncatalyzed.[...]
  
  $$\ce{2NaOCl -&gt; NaClO2 + NaCl}$$ 
  $$\ce{NaOCl + NaClO2 -&gt; + NaClO3 + NaCl}$$ 
  $$\ce{2OCl- -&gt; 2Cl- + O2}$$


But since this paper is in the 50's, most of the information would  be outdated or misleading. So, here is the result.

Wikipedia says:


  
  It decomposes (autoxidizes) when heated, forming sodium chlorate    and sodium chloride:
     $$\ce{3 NaOCl(aq) → NaClO3(aq) + 2 NaCl(aq)}$$
  Sodium hypochlorite decomposes with increasing temperature and under    the influence of light and such metals as copper, nickel, or
  cobalt:
     $$\ce{2 OCl⁻(aq) → 2 Cl⁻(aq) + O2(g)}$$
  


There are two pathways of decomposition of bleach. One is forming chlorate and other one is forming chloride. But the 1st reaction is dominant one i.e chlorate is the major product. But the formation of chlorate actually requires heat(to be precise 303-323 K).

The 2nd reaction also requires heat(343 K) and also some catalysts:


  Typical metals for nickel and copper are in the range of 50 ppb or
  less when sub-micron filtration is provided but many times, levels in
  the range of 300-400 ppb for nickel and 50-60 ppm for copper can be in
  the product which will create $\ce{O2}$ and increased decomposition.(source)


So, the conclustion is that the formation of chlorate or chloride do require heat. But there are 2 reaction that indeed happens when bleach is remained untouched for a long time i.e reaction with carbon dioxide and moisture.



  If not properly stored in airtight containers, sodium hypochlorite
  reacts with carbon dioxide to form sodium carbonate:
  
  $$\ce{2 NaOCl(aq) + CO2(g) → Na2CO3(aq) + Cl2(g)}$$


  $$\ce{NaClO + CO2 + H2O → NaHCO3 + HClO}$$
  
  Sodium hypochlorite react with carbon dioxide and water (moisture) to
  produce sodium bicarbonate and hypochlorous acid. Sodium hypochlorite -
  saturated solution. The reaction proceeds at room temperature.(chemiday)



Hypochlorous acid is somewhat unstable so it might decompose to give chlorine fumes or further react with hypochlorite to form chlorate.

$$\ce{OCl- + 2HOCl -&gt; ClO3- + 2HCl}$$